An integrated circuit card generally comprises a body having an integrated circuit incorporated therein, which circuit contains a processor unit associated with random access memory (RAM) and with data exchange means for exchanging data with an external device, such as a card reader. By way of example, the data exchange means comprise conductive areas for co-operating with electrodes of the external device, or a transceiver antenna for exchanging electromagnetic signals with an antenna of the external device. The processor unit, a microprocessor, performs operations on the data, and more particularly cryptographic calculations, that serve in particular to enable the processor unit to verify whether it is authorized to engage in a transaction with the external device to which it is connected by the exchange means. The RAM is used to store both data that is exchanged with the external device, and internal operating data such as intermediate results that are reused in the operations performed by the processor unit.
Such cards sometimes contain information that is confidential, such as a secret code, or they give access to premises or to equipment containing such information, and they are sometimes subjected to attempts at fraud that seek to disturb the operation of the processor unit in order to cause it to communicate confidential data with the outside or to reveal how it operates.
Such attempts at fraud are generally performed by making use of the RAM either in order to cause scraps of instructions to be stored therein, scraps that are inoffensive on their own, but that the processor unit will assemble and thereby constitute instructions that endanger the security of the card; or in order to recover therefrom information that the processor unit stores temporarily therein and that is not normally intended to be transmitted to the outside.